The present invention relates to a developing apparatus for an image recorder and, more particularly, to a developing apparatus installed in an electrophotographic copier or the like and including a developer transport member which carries one component magnetic toner, or developer, toward a predetermined developing region.
A developing apparatus used with, for example, an electrophotographic copier functions to supply toner, or developer, to an electrostatic latent image which is formed on a photoconductive drum or like image carrier so as to develop the latent image. Among some different types of developing apparatuses, an apparatus which uses one component magnetic toner is usually constructed such that the toner is magnetically deposited on a developer transport member having a magnet body as typified by a developing sleeve and is carried by the transport member to a predetermined developing region. For example, in a developing apparatus disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 50-45639, a cylindrical developing sleeve which serves as a developer transport member is located in the vicinity of a photoconductive drum or like image carrier and has therein a magnet body having a plurality of magnetic poles arranged in an annular configuration. While one component magnetic toner is deposited on the developing sleeve by the magnetic field which is developed by the magnet body, the developing sleeve and the magnet body are rotated relative to each other to transport the toner. In this instance, the toner is charged by the frictional force which is developed between toner particles and the developing sleeve when the toner particles are sequentially moved on the surface of the sleeve while rolling on that surface.
In a developing apparatus of the kind described, should the nearby poles of the magnet body be greatly spaced apart from each other, toner particles would fail to be deposited on the developing sleeve thin and uniformly. In the light of this, there has been proposed a developing apparatus in which a magnet body is magnetized to have different magnetic poles which alternate with each other at a predetermined small distance. Such an apparatus is capable of forming a thin layer of toner particles because the numerous different magenetic poles which are close to each other allow a small amount of toner particles to be surely deposited on the individual poles. However, a decrease in the distance between nearby poles is accompanied by a decrease in the magnetic field which is developed between the nearby poles. Should the magnetic field be decreased to fail to reach the outside of the developing sleeve, the magnet body would become unable to magnetically attract the toner particles and, therefore, to produce an even toner layer. Although a magnet member with magnetic poles which alternate with each other at a small distance may be driven to rotate at a high speed so as to provide an even toner layer as has already been proposed, the high speed rotation cannot be implemented without a driving mechanism which is apt to produce noise. Another drawback is that the high speed rotation of the magnet body generates a vortex current on the surface of the developing sleeve resulting in the generation of heat.
In another prior art developing apparatus, the magnet body disposed in the developing sleeve is replaced with a charging member in the form of a blade which is pressed against the surface of a developing sleeve. The charging blade serves to frictionally charge toner due to its pressing contact with the developing sleeve while thinning a toner layer to a predetermined thickness. With the charging blade only, however, the apparatus cannot cause a sufficient amount of toner to be forced into the pressing portion of the blade because toner is not deposited by a sufficient force on the developing sleeve, resulting in the friction being insufficient.